Master Bash Conditionals, Loops, and Signal Traps: Practical Scripts Explained

Condition Selection and Judgment

1. if statement

Usage format:

if condition1 ; then
  # commands for true condition
elif condition2 ; then
  # commands for true condition
else
  # commands when all conditions are false
fi

The script evaluates each condition sequentially; the first true condition executes its block and then exits the if structure.

Example – Determine age:

# Determine age
#!/bin/bash
read -p "Please input your age: " age
if [[ $age =~ [^0-9] ]]; then
  echo "please input a int"
  exit 10
elif [ $age -ge 150 ]; then
  echo "your age is wrong"
  exit 20
elif [ $age -gt 18 ]; then
  echo "good good work,day day up"
else
  echo "good good study,day day up"
fi

Analysis: The script first checks for non‑numeric characters, then validates the range, and finally distinguishes adults from younger users.

Example – Determine score:

# Determine score
#!/bin/bash
read -p "Please input your score: " score
if [[ $score =~ [^0-9] ]]; then
  echo "please input a int"
  exit 10
elif [ $score -gt 100 ]; then
  echo "Your score is wrong"
  exit 20
elif [ $score -ge 85 ]; then
  echo "Your score is very good"
elif [ $score -ge 60 ]; then
  echo "Your score is soso"
else
  echo "You are loser"
fi

Analysis: The script validates numeric input, then checks for out‑of‑range values, and finally categorises the score into different performance levels.

2. case statement

Usage format:

case $variable in
  PATTERN1) cmd ;; 
  PATTERN2) cmd ;; 
  *) cmd ;; 
esac

The case command supports glob‑style patterns such as *, ?, [], and a|b.

Example – Yes/No decision:

# Yes or No
#!/bin/bash
read -p "Please input yes or no: " ans
case $ans in
  [yY][eE][sS]|[yY]) echo yes ;;
  [nN][oO]|[nN]) echo no ;;
  *) echo false ;;
esac

Analysis: Accepts various capitalisations of “yes” and “no”; any other input yields “false”.

Four Loop Constructs

1. for loop

Usage formats:

# Iterate over a list
for name in list; do
  # loop body
 done

# C‑style for loop
for (( exp1; exp2; exp3 )); do
  # loop body
 done

Example – Sum of 1 to n:

# Sum of 1..n
#!/bin/bash
sum=0
read -p "Please input a positive integer: " num
if [[ $num =~ [^0-9] ]] || [ $num -eq 0 ]; then
  echo "input error"
  exit 1
else
  for i in $(seq 1 $num); do
    sum=$((sum + i))
  done
  echo $sum
fi

Analysis: Validates the input, then iterates from 1 to the entered number, accumulating the total.

2. while loop

Usage format:

while condition; do
  # loop body
 done

Special usage – reading a file line by line:

while read line; do
  # process $line
 done < /path/to/file

Example – Sum of odd numbers up to 100:

# Sum of odd numbers up to 100
#!/bin/bash
sum=0
i=1
while [ $i -le 100 ]; do
  if [ $((i % 2)) -ne 0 ]; then
    let sum+=i
  fi
  let i++
 done
 echo "sum is $sum"

Analysis: Iterates while the counter is ≤ 100, adds the number to the sum only when it is odd.

3. until loop

Usage format (inverse of while):

until condition; do
  # loop body
 done

Example – Monitor a user and kill the session when they log in:

# Monitor user xiaoming
until pgrep -u xiaoming &>/dev/null; do
  sleep 0.5
 done
pkill -9 -u xiaoming

Analysis: Checks every half second; when the user appears, the script terminates all processes belonging to that user.

4. select loop (menu)

Usage format:

select var in list; do
  # commands for chosen item
 done

Example – Simple menu with price display:

# Menu example
#!/bin/bash
PS3="Please choose the menu: "
select menu in mifan huimian jiaozi babaozhou quit; do
  case $REPLY in
    1|4) echo "the price is 15" ;;
    2|3) echo "the price is 20" ;;
    5) break ;;
    *) echo "no the option" ;;
  esac
done

Analysis: Displays numbered options, executes the corresponding case branch, and exits when the user selects “quit”.

Loop Control Statements

Commands: continue [N] – skip the rest of the current loop iteration and start the next one (N specifies the nesting level). break [N] – exit the loop early (N specifies the nesting level).

Example – Sum of odd numbers excluding 51:

# Sum 1+3+...+49+53+...+100
#!/bin/bash
sum=0
for i in {1..100}; do
  [ $i -eq 51 ] && continue
  [ $((i % 2)) -eq 1 ] && let sum+=i
 done
 echo sum=$sum

Analysis: Skips the iteration when i equals 51, otherwise adds odd numbers to the sum.

Parameter Shifting with shift

shift

removes the leftmost positional parameter, moving the remaining ones one position to the left.

Example – Create multiple users from arguments:

# Create users from arguments
#!/bin/bash
if [ $# -eq 0 ]; then
  echo "Please input at least one username"
  exit 1
else
  while [ -n "$1" ]; do
    useradd "$1" &>/dev/null
    shift
  done
fi

Analysis: Loops over all supplied arguments, creating a user for each name.

Signal Trapping with trap

Format:

trap 'command' SIGNAL   # Execute command when SIGNAL is received
trap '' SIGNAL          # Ignore SIGNAL
trap - SIGNAL          # Reset SIGNAL to default behavior
trap -p                 # List current traps

Common signals:

SIGHUP – reload configuration

SIGINT – interrupt (Ctrl+C)

SIGQUIT – quit (Ctrl+\)

SIGKILL – force kill (cannot be trapped)

SIGTERM – termination request

SIGCONT – continue after stop

SIGSTOP – stop (cannot be trapped)

Example – Prevent Ctrl+C from terminating a counting loop:

# Prevent Ctrl+C from stopping the loop
#!/bin/bash
trap 'echo press ctrl+c' 2
for ((i=0;i<10;i++)); do
  sleep 1
  echo $i
done

Analysis: The script catches signal 2 (SIGINT) and prints a message instead of exiting.

Example – Allow Ctrl+C after the first three iterations:

# Allow termination after three iterations
#!/bin/bash
trap '' 2               # Ignore SIGINT
for ((i=0;i<3;i++)); do
  sleep 1
  echo $i
 done
trap '-' SIGINT        # Restore default handling
for ((i=3;i<10;i++)); do
  sleep 1
  echo $i
 done

Analysis: The first loop ignores SIGINT; after it finishes, the trap is removed so the script can be stopped with Ctrl+C.

Additional Handy Bash Snippets

Random character generation

# Generate 8 random alphanumeric characters
cat /dev/urandom | tr -dc '[:alnum:]' | head -c 8

Random number

# Generate a random number between 0 and 6
echo $((RANDOM % 7))
# Generate a random number between 31 and 37
echo $((RANDOM % 7 + 31))

Colored output

# Red text
echo -e "\033[31malong\033[0m"
# Bold red text
echo -e "\033[1;31malong\033[0m"
# Red background
echo -e "\033[41malong\033[0m"
# Blinking red text
echo -e "\033[31;5malong\033[0m"

9×9 multiplication table

# 9×9 multiplication table
#!/bin/bash
for a in {1..9}; do
  for b in $(seq 1 $a); do
    c=$((a*b))
    echo -e "${a}x${b}=${c}\t\c"
  done
  echo
 done

Colorful isosceles triangle

# Colorful triangle
#!/bin/bash
read -p "Please input a num: " num
if [[ $num =~ [^0-9] ]]; then
  echo "input error"
else
  for i in $(seq 1 $num); do
    stars=$((2*i-1))
    for _ in $(seq 1 $((num-i))); do echo -n " "; done
    for _ in $(seq 1 $stars); do
      color=$((RANDOM%7+31))
      echo -ne "\033[1;${color};5m*\033[0m"
    done
    echo
  done
fi

Chessboard pattern

# Chessboard pattern
#!/bin/bash
red="\033[1;41m  \033[0m"
yellow="\033[1;43m  \033[0m"
for i in {1..8}; do
  if (( i % 2 == 0 )); then
    for _ in {1..4}; do echo -n -e "${red}${yellow}"; done
  else
    for _ in {1..4}; do echo -n -e "${yellow}${red}"; done
  fi
  echo
 done